Preparation of aluminum phosphates



Feb. 1, 1949. H. H. GREGER PREPARATION OF ALUMINUM PHOSPHATES Filed July 15,' 1945 n QN bw SSQNNS .Nu

Patented Feb.V 1, 1949 UNITED STATESY PATENT OFFICE Y 2,460,344 rnaranafriou oF ALUMINUM PnosPHATEs Herbert H. Greger, Washington, D. C. application July 13, 1943, serial No. 494,527

claims. (o1. 23-1o5) This invention relates toraluminum phosphates and more particularly has reference to a method of preparing soluble aluminum,phosphatesrwhich are solid at room temperature. l

This invention is directed to the preparation of aluminum phosphates of the type described in mycopendingY application Serial No. 490,495, filed June ll, 1943, nowforfeited.

I have found that solid aluminum phosphates can be prepared by reacting aluminum hydrate in finely divided form with phosphoric acid of high concentration. If the reaction is not controlled, however, difficulty is encountered in that the product sets rapidly into a hard mass which may be insoluble.

The difficulties in the preparation of the water soluble solid form of aluminum phosphates, particularly those close to and above the sesqui-phosphates, are largely due to their colloidal nature which is related to the water of hydration in the compound. Only the phosphates having a low aluminum content, such as the mono-aluminum phosphate, can be nearly free Iof water and still remain soluble. They must be nearly free from Water before they will solidify and even after solidication such compounds are very ductile. On the other hand, higher aluminum phosphates, such as the di-aluminum phosphate, may contain as much as 25% water and yet be in the form of a hard, brittle solid at room temperatures. If the higher aluminum phosphates, that is, those above the sesqui-aluminum phosphate, are formed with a very low Water content, it will be found that the product may no longer be fully solublev in water.

An object of this invention is to provide a method of preparing soluble aluminum phosphates which are solid at room temperatures.

Another object of this invention is to provide a method of preparing soluble aluminum phosphates which are solid at` room temperature, which involves controlling the speed of reaction to avoid excessive removal of the water content from the reaction mass,

A further object of this invention is to provide a method of making soluble aluminum phosphates which are solid at room temperatures by carrying out the reaction in stages.

Still another object of this invention is to provide a method of preparing soluble aluminum phosphates which are solid at room temperatures by reacting phosphoric acid and an aluminum containing compound to form a mono-aluminum y phosphate and then reacting the mono-aluminum 2 phosphate with iinely divided aluminum hydrate to form a higher aluminum phosphate.

It is also an object of this invention to control the temperature of reaction between phosphoric acid and an aluminum compound to maintain a predetermined water content in the reaction mass. In addition, the present invention has as one of its objects a method of preparing soluble aluminum Vphosphates which are solid at room temperature by controlling the Water content of the reactants and the water produced in the reaction to obtain a product having a definite water content.

With these and other objects in view, which will appear more fully hereinafter, the present invention resides in the method hereinafter set forth and the steps followed in carrying out the same.

`In the drawings, the single gure is a diagram showing the physical state of aluminum phosphates in relation to the content of free water and composition.

The phosphates to which the present invention relates are those comprised between mono-aluminum phosphate and di-aluminum phosphate, in which the ratio of aluminum to the P04 radical ranges from 1:3 for mono-aluminum phosphate, A1(H2PO4)3, to 2:3 for di-aluminum phosphate, Al2(I-IP04)3. Many of the phosphates lying at intermediate points in this rangerhave highly desirable properties and are valuable products. The particular compound that is formed depends upon the proportions of the materials which enter into the reaction for the formation of the aluminum phosphate.

The various aluminum phosphates ranging from monoto di-aluminum phosphates may exist in the liquid or solid phase depending upon Vthe amount of water that is present and the temperature of the product. As illustrated in the ,single figure of the drawings, a mono-aluminum phosphate designated A-i, that is, an aluminum phosphate in which the ratio of aluminum to P04 is 1:3, will be at 20 C. a ductile solid when the water content is substantially zero, An aluminum phosphate, such as as the sesquialuminum phosphate, in which the ratio of aluminumV to P04 is 11/2 :3 and designated on the drawings as A-l 1/2, will be solid at 20 C. with a water content of over 25%. The solid oli-aluminum phosphate designated on the drawing as A-2 may have, at 20 C., a water content of about 35%.

The curve I shown on the drawing represents a zone or line of demarcation between liquid or 3 ductile solid and hard solid aluminum phosphates at 20 C. for phosphates lying between monoand di-aluminum phosphates at various water contents. The slightly inclined, subst-ain tially horizontal dotted lines represent the water content of aluminum phosphates prepared from phosphoric acids of various concentrations. For instance, it will be noted that when an aluminum phosphate is prepared from almninurn hydrate with 100% phosphoric acid, the product will have a water content low enough so that for most of the products it will be in the solid phase. On. the other hand, when an aluminum phosphate is made from aluminum hydrate and phosphoric acid 0i 75% concentration, it will be noted that all of the products are in the liquid phase. This is due to the relatively high watercontent which is introduced into the product from the dilution of the phosphoric acid.

The water content originates from the content of free Water inthe acid and from the water of reaction. The latter is important and must not be overlooked as in some casesit may be larger than the amount of free water carried into the compound from the acid.

From the monoto the sesquito the cli-'aluminum phosphate, the Aproperties change as the aluminum to phosphate ratio increases. For instance, the mono-aluminum phosphate the compositions close toit have a relatively good solubility vin water even when dehydrated at a temperature 'of 360 F. while the solubility of the higher phosphates may -be considerahly impaired or even'destroyed by heating drying at this temperature. 'Y

En, order to understand the solubility of the sesquiand of the diealuminum phosphates, it is necessary to consider their colloidal nature. ln relatively concentrated solutions Vwhere hydrolysis is essentially absent, thesephosphates have the properties'of colloids. The colloidal state of these' solutions is retainedin thersolids if'water is gradually removed from lsuch solutions by drying at moderatetemperatures below aboutY 59C. and hydrated solids are formed. These will disperse in water and form clean'viscous solutions by virtue of their 'colloidal structure. if this structure is destroyed,the resulting solids are forthe'most part insoluble in water.

It is very interesting to note that at room temperature the solid state is obtained long before all water is removed. This water is of importance for preserving the solubility of the higher phosY phates and the Vpercentage of water retained varies with the alumina-phosphate ratio for a given degree of hardness. For example, at a point where pulverizing can be readily accomplished, the sesqui-aluminum phosphate may contain about 25%, the di-aluminum phosphate as much as or considerably more than 25% of Water oi hydration.

The solid aluminum phosphates in their hydrated form are essentially an extension ci tl e liquid state or vice versa. By careful control of the water content, it is possible to produce any desired viscosity whether the starting materials are the liquid or the solid aluminum phosphates.

The latter have the appearance of a hard, translucent resin-like material of conchoidal fracture. When heated, they become soft and finally melt to a viscous fllid. When rapidly heated and the temperature reaches the range above 230 to 250 F., depending on the type of phosphate, the vapor tension increases to above atmospheric and the melted material bloats and froths until the dehydration has progressed to the point where it solidi'es. This residue is not fully inert to water but the colloidal state cannot be recovered Without .re-processing.

For commercial purposes, the aluminum phosphates can be handled in many cases much more conveniently in solid than in liquid form. This is trueV not only for shipping purposes but also in actual product manufacture. A finely divided solid can in many cases be mixed very effectively with other dry', powdered substances. Water may subsequently be introduced in the necessary amount.

' There is an essential difference in the production requirements of a liquid and a solid aluminum phosphate. As the water content becomes smaller, such factors as the control of mixing, temperature and rate of reaction and the regulation of the exact water `content in the iinal product becomes more and more difficult.

Several factors have a bearing on this situation. First of all, the aluminum hydrate is not a trong base and from the dissociation constants of the secondary and the tertiary hydrogen of the phosphoric acid, it will be seen that once the mono-aluminum phosphate is formed the remaining hydrogens belong to a relatively Weak or sluggishly reacting acid. For this reason, it is necessary tovuse, .in the production of the higher phosphates, a very iinely divided aluminum hydrate and to get it intimately and very uniformly distributed with respect to the phcsa phoric acid or the phosphoric anhydride, if this is used in place of the acid. The .nely divided state and intimatecontact will of necessity prou mote the rate of reaction and consequently a large amount of heat of reaction will beliberated in a very brief space of time. This inrturn will have the undesired eiect that some of the water intended as a constituent of the iinal product will be boiled out or lost and the water content will become indefinite. The .product may then set up when still hot into Va hard mass almost immediately and it may be quitedifiicult Vto removeit from the reaction vessel. .For all practicalrpurposes, it will be nearly impossible to obtain the necessary perfect uniformity. In mixing a very finely divided base and acid together, even ii the temperature is held .down by cooling with water, the reaction usually get under wayand out of hand before the mixing is complete. This again results in a non-homogeneous product containing lumps of unreacted base and a tacky liquid de ficient in aluminum which will not solidify.

It is-therefore important to control the reaction rate in order to gain the necessary time for complete mixing. Aside from this,.it is necessary to keep the final reaction temperature down for reasons that will become apparent later.

In some instances, it will be necessary to establish the limits of Water content because it has been found that for some purposes solid aluminum phosphates which have been dried to too great an extent will not be as useful as `the products having-the maximum permissible water content for 'comminuting, or a somewhat lower water content. While it may be unimportant in a number of cases to produce an aluminum phos phate having the maximum permissible water content for the solid state, .i. e. 4aluminum phosphates lying justbeloW the borderline indicated by reference character l in thedrawing, for such purposes as plastid'hot molding compositions, it seems desirable to form solid aluminum phosphates having the maximum practical Water 5, content permissible inthe solid phase at room temperature.

. The solid state also depends on the temperaturev of the phosphate, and of course a high iluidity ofthe phosphate at a given temperature will dependv on both Water content and composition with a tendency towardk increased viscosity in` reaction is effected between an aluminum com-V pound and phosphoric acid in a plurality of stages. If desired, the reaction can be started with phosphorous pentoxide. In this instance, the phosphorous pentoxide is converted first yto the acid 'by the addition of the desired amount of water. After cooling the acid, the monoaluminum phosphate is formed 'by effecting a reaction between the acid and an aluminum compoundv such as aluminum hydrate, aluminum oxide (alumina) f, bauxite, or calcined clay.

It hasbeen found that if the aluminum material is of suicient coarseness, the reaction between the same and the phosphoric acid will be considerably reduced after the formation of mono-aluminum phosphate. To carry the reaction further andform higher aluminum phosphates up to the di-aluminum phosphate, it is necessary to utilize finely divided aluminous material fine enough to give suicient contact sur face for the reaction to proceed further. Aluminum hydrate having a particle size of between about .3 and about .6 micron is suitable for this purpose. In addition, it will be found necessary to heat the mono-aluminum phosphate and the finely divided aluminum hydrate for several hours in a closed container at about 100 C. The resulting phosphate is glassy clear and by adjusting the vinitial water content of the acid, solid phosphate may be produced. The water content in the product originates from the content of the free water in the acid and from the waterY produced as a result of the reaction. The latter is important and should not be overlooked as Ainsome cases it will be larger than the amount of free water carried into the compound by the acid.

It is, ofcourse, possible to combine the formation of acid and mono-aluminum phosphate into one single operation. The amount of heat generated during the reaction is sufficient to vaporize out a certain amount of water which can be restored. For the production of the mono-aluminumphosphate, the grain size of the aluminum hydrate is not of particular importance except it should be relatively coarse, that is, of a size that will pass through 100 to 200 mesh which will prevent an excessively rapid reaction and give time enough for even mixing. Such relatively coarse grain, however, cannot be used in* the preparation of the phosphates with a higher aluminum content than the mono-aluminum phosphate. In this step the aluminum hydrate should be extremely fine and for this purpose aluminum hydrate No. C730 of the Aluminum Company of America having a particle size of between .3 to .6 micron is satisfactory. If this hydrate were mixed directly with the acid, the reaction vwould proceed so rapidly that uniformmixing would be impossible and a lumpy non-homogeneoussubstance would result.

phosphate first from relatively coarse hydrate and in another step using the Iinely divided aluminum hydrate to form the `higher phosphates.

VMono-aluminum phosphate solution is a clear,

slightly yellowish, syrupy liquid and the aluminumV hydrate No. C730 readily mixes with the monoaluininum phosphate and, when heated to 100 C., the fine hydrate is dissolved. f f

A mono-aluminum phosphate may be prepared,

for instance, from three mols of phosphorus pentoxide or 4265grams, 156 grains of aluminum hydrate and 108 grams of water which is only enough water to form the mono-aluminum phosphate without water of hydration. If this ma-` terial is allowed to cool to room temperature, it will form a somewhat ductile solid which may be placed at the zeroV point of the diagram of the attached drawing,

If this material is prepared from an equivalent amount or 6 mols of phosphoric acid and heated with nely dividedaluminum hydrate having a particle size ranging between .3 and .6 of a mi. cron at a temperature of about 100 C. in a closed vessel, the finely divided aluminum hydrate will be dissolved and a reaction 4will takeplace .resulting in the formation of a higher aluminum `phosphate depending upon the proportions of the material entering the reaction. If this material is allowed to cool to room temperature, it will be solid provided the total Water resulting from the reaction and the water content of the acid is not too great for the particular chemical composition formed. In other words, if the materiallies below the line or zone indicated by curve I inthe drawings, the material will be in solid form.

AA mono-aluminum phosphate may be prepared by reacting 85% phosphoric acid (I-IaPOi) withv v Of the 212 parts of water in the mono-aluminum resulting mass is heated to about 100 C. in aL f phosphate, about 104 parts were introduced as water` of dilution of the acid and approximately 108 parts were formed by the reaction between Ythe phosphoric acid and the aluminum hydrate.

To prepare an aluminum phosphate in which the aluminum to phosphate ratio is lizB, there should be added to about 848 parts of the prepared mono-aluminum phosphate, about 39 parts of aluminum hydrate (AlzOsHzO') in nely divided form.

It has been found that a finely divided aluminum hydrate having a particle size ranging between .3 and .6 of a micron is suitable for entering into a reaction ywith mono-aluminum phosphate to form aluminum phosphates having higher aluminum concentrations. After thoroughly mixing the nely divided aluminum hydrate and the mono-aluminum phosphate, the

closed vessel. After the reaction is complete, the aluminum phosphate formed will exist in the form of a very heavy viscous liquid at room tem perature. As will be noted from the chart shown in the drawing, an aluminum phosphate pre'- pared as described above and having an aluminum to phosphate ratio of 1%:3 will contain between 26 and 27% water.

' If there is addedto 848 parts by Weight of the It visV therefore necessary to form the mono-aluminum' amasar mono-aluminum phosphate, :about .'28 iparts Vby` weight of 'finely divided aluminum hydrate .having a particle size ,ranging between .3 and .6 .of a micron under the conditions specified hereinbefore, i. e.'by'heatingto.about 100" Cdnaclosed vessel, an aluminum phosphate `having an aluminum to .phosphate ratio vof l/Zr, will be formed. This product is in the form of a ductile solid at room temperatureand contains slightly more than 28% of water.

By rreacting 848 parts by y*weight ofthe monoaluminumphosphate with.97.5 parts by Weight of .the .finely divided aluminum hydrate having a particle size of .3 to .6 micron, under reaction conditions as specied above, an aluminum phosphate having an aluminum to phosphate ratio of of water of `hydration remains `in `*the :phosphate While in the foregoing description it has :been indicated that .the .reaction'may beibroken down into two stages, therst of which results in 'the formation of a rmono-aluminum phosphate .and the second of which involves the .reaction between the so formed .mono-aluminum 'phosphate and nely divided aluminum hydrate, it should be noted'that it is within the concept of the present invention to form an aluminum phosphate having a lower aluminum content than is -represented' by the Vmono-aluminum phosphate, for

instance, analuminum phosphate having analu-j Y mina-phosphate ratio of 1/213 may be rst formed 1%:3will be formed. "Ihismaterial'will be a definite hard solid vbody andas indicated on the chart of the drawing, this material will have a water content of about 291/271.

An aluminum phosphate having an aluminum Vto phosphate ratio of '1%:3 may be prepared by reacting 848 parts by weight of the `mono-aluminum phosphate with about 117 lparts by Weight of the nely .divided aluminum hydrate having a particle size between .3 and .6 micron under the reaction yconditions specied above. rIhis material at room temperature is a hard brittle solid and as indicated inthe 'chart oi the drawing has a water content of between 30 and 31%.

`The dotted lines having the captions 90, 95 and 100% indicate the water content of the various aluminum phosphates prepared from acids of these concentrations provided Ynone ofthe Waterv introduced into the phosphate through the `acid is allowed to escape during the reaction and provided no waterfroma-n extraneous source is added.

Y From the foregoing, it will be noted that an aluminum -phosphate having an aluminum toY phosphate radical ratio of 11/413 is a very Aheavy liquid at room temperature; the aluminum phosphate having an aluminum to phosphate .ratio of 11/2:3, i. e. the sesqui-phosphate is a slightly ductile solid at room temperature. On the other hand, -the aluminum phosphate having Van aluminum to phosphate ratio above 11/2z3, i. e. 1%:3 or 1%:3 and the higher aluminum Yphosphates up to the di-aluminum phosphate are quite hard and brittle. All ofthe aluminum phosphates except the di-aluminum phosphate are clear and fully reacted and all except the dialuminum phosphate bloat considerably on quick heating. VThe sesqui-phosphate prepared from the 85% phosphoric acid containing about 281/2070 free water or water of hydration is too close to the edge of the soft state to be grindable without proper precautions. Of course, if this material is dried to reduce the water content to bring the material Well below the linev represented by curve l in the drawing, it will be converted into a hard, brittle substance. If a sesqui-aluminum phosphateris prepared from 90% phosphoric acid, a hard, brittle substance is formed. On the other hand, if an aluminum phosphate having an aluminum to phosphate ratio of 1%:3 is prepared with 85% phosphoric acid, a substance will be formed which is hard and brittle at room temperature. Y

The water in the various Yphosphates is held tenaciously at temperatures -near the boiling point of water and it is necessary to increase the temperature .to about D-250 F. for eliminating a part or all of the water. VIn some instances, even when heated to ,300 F.,-ancertain amount and 'this materialreacted Withinely divided aluminum hydrate-to form 'higher-aluminum phosphate. Thus, by first forming an aluminum phosphate having a low alumina content and later reacting this phosphate with Hfurther aluminum hydrate to form an aluminum phosphate having a high alumina contenuthe temperature oi the reactions can be vmore accurately controlled and the water contentof the resulting product determined. YThe aluminum to phosphate ratio of the :aluminum phosphate formed in the first stage may'varygbetween 1/2:3 and 1:-3, depending upon the will of the operator. Y

llin the foregoing description, vthe lterm room temperature is intended to mean those `temperatures occurring in plants where the product of this invention is made and used.

Generally, a given aluminum phosphate having a certain Water content and which is solid at a given temperature will be'solid at'lower temperatures but may not be solid at a'higher temperature. By reducing the-water content-of the material, it wilt remain solid even at higher temperatures; The aluminum phosphates which are soluble and are solid at room temperatures are very useful in the manufacture of various products in which :aluminum `phosphates act as binders, etc. VVThe present invention provides a simple and yet very satisfactory -method of controllingV the formation of soluble 'aluminum phosphates which are solid at room temperatures.

I claim: Y Y l Y l; Avmethod of making Water soluble aluminum phosphates up to di-aluminum phosphate solid at room temperatures-comprising reacting relatively coarse aluminum hydrate `withphosphoric acid in proportions `to form A1(H2PO4)3 and then reacting the Al(H2PO4)3 with finely-divided aluminum hydrate at a temperature of about C. for several hours to form higherV aluminum phosphates, and controlling the Water contentV of V the reactants and the reaction mass `to form` a solid water, soluble compound having aAwatercontentranging `from zero for the 1:3 alumina to .phosphate ratio to -slightlyover 25% for the 11/223 alumina to phosphate vratio to about35% for the 2:3 alumina to phosphate ratio. c

2. A method of making watersoluble aluminum phosphates up to .di-aluminum phosphate solid at room temperatures comprising reactingrelatively coarsealuminum hydrate wtih-phosphorio acid in proportions toform AMHQPOila'and then adding aluminum hydrate vof a much finer' grain size to form phosphatesof a higher aluminum content up todi-aluminum phosphate .and having' a .water-content ranging from .Zero for the 1:3 alumina to phosphate ratio to slightlyrover 25% forftheV 'l1/2:3 alumina to vphosphate lratio toabout 35% for the.2:3 alumina to phosphate ratio and controlling the temperature of the reaction mixture during the addition of the ne aluminum hydrate at about 100 C.

3. A method of making Water soluble aluminum I phosphates up to cli-aluminum phosphatersolid at room temperatures comprising reacting relatively coarse aluminum hydrate of a particle size ranging from 1GO-200 mesh with phosphoric acid in proportions to form AMI-1213003, and then adding aluminum hydrate of a much nner grain size to form phosphates of a higher aluminum content up to dialuminum phosphate and having a Water content ranging from zero for the 1:3 alumina to phosphate ratio to slightly over 25% for the 11/2z3 alumina to phosphate ratio to about 35% for the 2:3 alumina to phosphate ratio and controlling the temperature of the reaction mixture during the addition of the ne aluminum hydrate at about 100 C.

4. A method of making water soluble aluminum phosphates up to di-aluminum phosphate solid at room temperatures comprising reacting relatively coarse aluminum hydrate of a particle size ranging from D-200 mesh with phosphoric acid in proportions to form Al(H2PO4)3, and then adding aluminum hydrate of a particle size ranging from .3 to .6 microns to form phosphates of a higher aluminum content up to di-aluminum phosphate and having a Water content ranging from zero for the'1z3 alumina to phosphate ratio to slightly over 25% Vfor the 11/2t3 alumina to phosphate ratio to about 35% for the 2:3 alumina to phosphate ratio and controlling the temperature of the reaction mixture during the addition of the ne aluminum hydrate at about 100 C.

5. A method of making Water soluble aluminum phosphates up to cli-aluminum'phosphate solid at room temperatures comprising reacting relatively coarse aluminum hydrate oi a particle size ranging from 10G-200 mesh with phosphoric acid in proportions to form Al H2POi s, and then adding aluminum hydrate of a particle size ranging from .3 to .6 microns and heating to a temperature of about 100 C. to form phosphates of a higher aluminum content up to di-aluminum phosphate and having a Water content ranging from zero for the 1:3 alumina to phosphate ratio to slightly over` 25% for the 11/2z13 alumina to phosphate ratio tc about for the 2:3 alumina to phosphate ratio.

HERBERT H. GREG-ER.

REFERENCES CTED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,499,611 Gravell July 1, 1924 1,961,127 Coleman June 5, 1934 1,998,182 Anable Apr. 16, 1935 2,160,760 Knox May 30, 1939 2,161,290 Grimm June 6, 1939 OTHER REFERENCES Mellor- Comprehensive Treatise on Inorganic and Theoretical Chemistry, Vol. 5, Longmans, N. Y., 1924, page 365. 

